Occipital Transcranial Magnetic Stimulation Research Articles

Mapping the routes of perception: Hemispheric asymmetries in signal propagation dynamics.

The visual system has long been considered equivalent across hemispheres. However, an increasing amount of data shows that functional differences may exist in this regard. We therefore tried to characterize the emergence of visual perception and the spatiotemporal dynamics resulting from the stimulation of visual cortices in order to detect possible interhemispheric asymmetries. Eighteen participants were tested. Each of them received 360 transcranial magnetic stimulation (TMS) pulses at phosphene threshold intensity over left and right early visual areas while electroencephalography was being recorded. After each single pulse, participants had to report the presence or absence of a phosphene. Local mean field power analysis of TMS-evoked potentials showed an effect of both site (left vs. right TMS) of stimulation and hemisphere (ipsilateral vs. contralateral to the TMS): while right TMS determined early stronger activations, left TMS determined later stronger activity in contralateral electrodes. The interhemispheric signal propagation index revealed differences in how TMS-evoked activity spreads: left TMS-induced activity diffused contralaterally more than right stimulation. With regard to phosphenes perception, distinct electrophysiological patterns were found to reflect similar perceptual experiences: left TMS-evoked phosphenes are associated with early occipito-parietal and frontal activity followed by late central activity; right TMS-evoked phosphenes determine only late, fronto-central, and parietal activations. Our results show that left and right occipital TMS elicits differential electrophysiological patterns in the brain, both per se and as a function of phosphene perception. These distinct activation patterns may suggest a different role of the two hemispheres in processing visual information and giving rise to perception.

Transcranial magnetic stimulation to frontal but not occipital cortex disrupts endogenous attention

Covert endogenous (voluntary) attention improves visual performance. Human neuroimaging studies suggest that the putative human homolog of macaque frontal eye fields (FEF+) is critical for this improvement, whereas early visual areas are not. Yet, correlational MRI methods do not manipulate brain function. We investigated whether rFEF+ or V1/V2 plays a causal role in endogenous attention. We used transcranial magnetic stimulation (TMS) to alter activity in the visual cortex or rFEF+ when observers performed an orientation discrimination task while attention was manipulated. On every trial, they received double-pulse TMS at a predetermined site (stimulated region) around V1/V2 or rFEF+. Two cortically magnified gratings were presented, one in the stimulated region (contralateral to the stimulated area) and another in the symmetric (ipsilateral) nonstimulated region. Grating contrast was varied to measure contrast response functions (CRFs) for all attention and stimulation combinations. In experiment 1, the CRFs were similar at the stimulated and nonstimulated regions, indicating that early visual areas do not modulate endogenous attention during stimulus presentation. In contrast, occipital TMS eliminates exogenous (involuntary) attention effects on performance [A. Fernández, M.Carrasco,Curr. Biol.30, 4078-4084 (2020)]. In experiment 2, rFEF+ stimulation decreased the overall attentional effect; neither benefits at the attended location nor costs at the unattended location were significant. The frequency and directionality of microsaccades mimicked this pattern: Whereas occipital stimulation did not affect microsaccades, rFEF+ stimulation caused a higher microsaccade rate directed toward the stimulated hemifield. These results provide causal evidence of the role of this frontal region for endogenous attention.

Suppressing visual hallucinations in an adolescent by occipital transcranial magnetic stimulation: A single-case experimental research design

ABSTRACT Visual hallucinations after central or peripheral impairment, commonly called Charles Bonnet syndrome, are often highly distressing and with few available treatment options. Here we report a case where an adolescent developed severely distressing visual hallucinations after hypoxic damage to the occipital cortex following a suicide attempt. The patient received active and sham occipital continuous theta-burst stimulation (cTBS) in a single-case experimental research design and a subsequent open phase, to evaluate cTBS as a Charles Bonnet treatment. The visual hallucinations seemed to decrease more during active than sham cTBS in the blind phase, and in the following week of repeated five daily treatments they almost disappeared. A normalization of increased activity in the lateral visual network after cTBS was observed on a functional magnetic resonance imaging resting-state analysis compared with 42 healthy controls. Visual evoked potentials stayed largely unchanged both in the sham-controlled blind phase and the subsequent open phase. During the two weeks after the open phase with repeated cTBS sessions, the visual hallucinations gradually reappeared and almost returned to the baseline level. Our findings suggest that active cTBS over the primary visual cortex can reduce visual hallucinations through modulation of downstream visual regions, though the effect is temporally limited.

Seeing again: treatment of functional visual loss

There is very little published literature on treatment strategies for functional visual loss. We present two people with long duration of functional visual loss who achieved complete recovery with a...

Propofol-induced unresponsiveness is associated with impaired feedforward connectivity in cortical hierarchy

BackgroundImpaired consciousness has been associated with impaired cortical signal propagation after transcranial magnetic stimulation (TMS). We hypothesised that the reduced current propagation under propofol-induced unresponsiveness is associated with changes in both feedforward and feedback connectivity across the cortical hierarchy. MethodsEight subjects underwent left occipital TMS coupled with high-density EEG recordings during wakefulness and propofol-induced unconsciousness. Spectral analysis was applied to responses recorded from sensors overlying six hierarchical cortical sources involved in visual processing. Dynamic causal modelling (DCM) of induced time–frequency responses and evoked response potentials were used to investigate propofol's effects on connectivity between regions. ResultsSensor space analysis demonstrated that propofol reduced both induced and evoked power after TMS in occipital, parietal, and frontal electrodes. Bayesian model selection supported a DCM with hierarchical feedforward and feedback connections. DCM of induced EEG responses revealed that the primary effect of propofol was impaired feedforward responses in cross-frequency theta/alpha–gamma coupling and within frequency theta coupling (F contrast, family-wise error corrected P<0.05). An exploratory analysis (thresholded at uncorrected P<0.001) also suggested that propofol impaired feedforward and feedback beta band coupling. Post hoc analyses showed impairments in all feedforward connections and one feedback connection from parietal to occipital cortex. DCM of the evoked response potential showed impaired feedforward connectivity between left-sided occipital and parietal cortex (T contrast P=0.004, Bonferroni corrected). ConclusionsPropofol-induced loss of consciousness is associated with impaired hierarchical feedforward connectivity assessed by EEG after occipital TMS.

Occipital transcranial magnetic stimulation discriminates transient neurological symptoms of vascular origin from migraine aura without headache

The diagnosis of transient neurological attacks (TNA) relies on medical history. Transient ischemic attack is often considered until proven otherwise, because of lack of objective paraclinical tools that can help discriminate TIA from differential diagnoses such as migraine aura. This may lead to needless and potentially harmful stroke secondary prevention in many cases. This study aimed at determining the yield of occipital transcranial magnetic stimulation (oTMS) in discriminating TNA of vascular origin from migraine aura without headache (MAWH). Ten patients with acute TNA of vascular origin and ten patients with migraine aura without headache (MAWH), without prior history of migrainous headache, were prospectively included. TNA of vascular origin were considered for individuals presenting unilateral focal symptoms with full resolution within 24hours and positive diffusion weighted imaging (DWI+). For individuals with MAWH, diagnostic criteria were either ICHD-III beta or Fischer criteria for a first episode of MAWH. All participants underwent one session of oTMS. Induction and threshold of phosphene induction were recorded. In TNA of vascular origin, MRI disclosed cortical lesions in nine and one sub-cortical lesion. Phosphenes were induced in 9/10 subjects with MAWH with a mean threshold of 66% of the maximal intensity, whereas oTMS induced phosphenes in only one subject with TNA of vascular origin at a threshold of 85%. In this pilot study, oTMS was found to be an effective tool to discriminate between MAWH and transient neurological symptoms of vascular origin.

Distinct Oscillatory Frequencies Underlie Excitability of Human Occipital and Parietal Cortex

Transcranial magnetic stimulation (TMS) of human occipital and posterior parietal cortex can give rise to visual sensations called phosphenes. We used near-threshold TMS with concurrent EEG recordings to measure how oscillatory brain dynamics covary, on single trials, with the perception of phosphenes after occipital and parietal TMS. Prestimulus power and phase, predominantly in the alpha band (8-13 Hz), predicted occipital TMS phosphenes, whereas higher-frequency beta-band (13-20 Hz) power (but not phase) predicted parietal TMS phosphenes. TMS-evoked responses related to phosphene perception were similar across stimulation sites and were characterized by an early (200 ms) posterior negativity and a later (>300 ms) parietal positivity in the time domain and an increase in low-frequency (∼5-7 Hz) power followed by a broadband decrease in alpha/beta power in the time-frequency domain. These correlates of phosphene perception closely resemble known electrophysiological correlates of conscious perception of near-threshold visual stimuli. The regionally differential pattern of prestimulus predictors of phosphene perception suggests that distinct frequencies may reflect cortical excitability in occipital versus posterior parietal cortex, calling into question the broader assumption that the alpha rhythm may serve as a general index of cortical excitability.SIGNIFICANCE STATEMENT Alpha-band oscillations are thought to reflect cortical excitability and are therefore ascribed an important role in gating information transmission across cortex. We probed cortical excitability directly in human occipital and parietal cortex and observed that, whereas alpha-band dynamics indeed reflect excitability of occipital areas, beta-band activity was most predictive of parietal cortex excitability. Differences in the state of cortical excitability predicted perceptual outcomes (phosphenes), which were manifest in both early and late patterns of evoked activity, revealing the time course of phosphene perception. Our findings prompt revision of the notion that alpha activity reflects excitability across all of cortex and suggest instead that excitability in different regions is reflected in distinct frequency bands.

Right hemisphere dominance directly predicts both baseline V1 cortical excitability and the degree of top-down modulation exerted over low-level brain structures

Right hemisphere dominance for visuo-spatial attention is characteristically observed in most right-handed individuals. This dominance has been attributed to both an anatomically larger right fronto-parietal network and the existence of asymmetric parietal interhemispheric connections. Previously it has been demonstrated that interhemispheric conflict, which induces left hemisphere inhibition, results in the modulation of both (i) the excitability of the early visual cortex (V1) and (ii) the brainstem-mediated vestibular–ocular reflex (VOR) via top-down control mechanisms. However to date, it remains unknown whether the degree of an individual’s right hemisphere dominance for visuospatial function can influence, (i) the baseline excitability of the visual cortex and (ii) the extent to which the right hemisphere can exert top-down modulation. We directly tested this by correlating line bisection error (or pseudoneglect), taken as a measure of right hemisphere dominance, with both (i) visual cortical excitability measured using phosphene perception elicited via single-pulse occipital trans-cranial magnetic stimulation (TMS) and (ii) the degree of trans-cranial direct current stimulation (tDCS)-mediated VOR suppression, following left hemisphere inhibition. We found that those individuals with greater right hemisphere dominance had a less excitable early visual cortex at baseline and demonstrated a greater degree of vestibular nystagmus suppression following left hemisphere cathodal tDCS. To conclude, our results provide the first demonstration that individual differences in right hemisphere dominance can directly predict both the baseline excitability of low-level brain structures and the degree of top-down modulation exerted over them.

Occipital long-interval paired pulse TMS leads to slow wave components in NREM sleep.

Neural correlates of conscious vs unconscious states can be studied by contrasting EEG markers of brain activity between those two states. Here, a task-free experimental setup was used to study the state dependent effects of occipital transcranial magnetic stimulation (TMS). EEG responses to single and paired pulse TMS with an inter-stimulus-interval (ISI) of 100 ms were investigated under Non-REM (NREM) sleep and wakefulness. In the paired pulse TMS condition adopting this long ISI, a robust positive deflection starting around 200 ms after the second pulse was found. This component was not obtained under wakefulness or when a single TMS pulse was applied in sleep. These findings are discussed in the context of NREM sleep slow waves. The present results indicate that the long interval paired-pulse paradigm could be used to manipulate plasticity processes in the visual cortex. The present setup might also become useful for evaluating states of consciousness.

Waves of awareness for occipital and parietal phosphenes perception

Transcranial magnetic stimulation (TMS) of the occipital cortex is known to induce visual sensations, i.e. phosphenes, which appear as flashes of light in the absence of an external stimulus. Recent studies have shown that TMS can produce phosphenes also when the intraparietal sulcus (IPS) is stimulated. The main question addressed in this paper is whether parietal phosphenes are generated directly by local mechanisms or emerge through indirect activation of other visual areas. Electroencephalographic (EEG) signals were recorded while stimulating left occipital or parietal cortices inducing phosphene perception in healthy participants and in a hemianopic patient who suffered from complete destruction of the early visual cortex of the left hemisphere. Results in healthy participants showed that the onset of phosphene perception induced by occipital TMS correlated with differential cortical activity in temporal sites while the onset of phosphene perception induced by parietal TMS correlated with differential cortical activity in the stimulated parietal site. Moreover, IPS-TMS of the lesioned hemisphere of the hemianopic patient with a complete lesion to V1 showed again that the onset of phosphene perception correlated with differential cortical activity in the stimulated parietal site. The present data seem thus to suggest that temporal and parietal cortices can serve as different local early gatekeepers of perceptual awareness and that activity in the occipital cortex, although being relevant for perception in general, is not part of the neural bases of the perceptual awareness of phosphenes.

Occipital TMS at phosphene detection threshold captures attention automatically

Strong stimuli may capture attention automatically, suggesting that attentional selection is determined primarily by physical stimulus properties. The mechanisms underlying capture remain controversial, in particular, whether feedforward subcortical processes are its main source. Also, it remains unclear whether only physical stimulus properties determine capture strength. Here, we demonstrate strong capture in the absence of feedforward input to subcortical structures such as the superior colliculus, by using transcranial magnetic stimulation (TMS) over occipital visual cortex as an attention cue. This implies that the feedforward sweep through subcortex is not necessary for capture to occur but rather provides an additional source of capture. Furthermore, seen cues captured attention more strongly than (physically identical) unseen cues, suggesting that the momentary state of the nervous system modulates attentional selection. In summary, we demonstrate the existence of several sources of attentional capture, and that both physical stimulus properties and the state of the nervous system influence capture.

Enhanced awareness followed reversible inhibition of human visual cortex: a combined TMS, MRS and MEG study.

This series of experiments investigated the neural basis of conscious vision in humans using a form of transcranial magnetic stimulation (TMS) known as continuous theta burst stimulation (cTBS). Previous studies have shown that occipital TMS, when time-locked to the onset of visual stimuli, can induce a phenomenon analogous to blindsight in which conscious detection is impaired while the ability to discriminate ‘unseen’ stimuli is preserved above chance. Here we sought to reproduce this phenomenon using offline occipital cTBS, which has been shown to induce an inhibitory cortical aftereffect lasting 45–60 minutes. Contrary to expectations, our first experiment revealed the opposite effect: cTBS enhanced conscious vision relative to a sham control. We then sought to replicate this cTBS-induced potentiation of consciousness in conjunction with magnetoencephalography (MEG) and undertook additional experiments to assess its relationship to visual cortical excitability and levels of the inhibitory neurotransmitter γ-aminobutyric acid (GABA; via magnetic resonance spectroscopy, MRS). Occipital cTBS decreased cortical excitability and increased regional GABA concentration. No significant effects of cTBS on MEG measures were observed, although the results provided weak evidence for potentiation of event related desynchronisation in the β band. Collectively these experiments suggest that, through the suppression of noise, cTBS can increase the signal-to-noise ratio of neural activity underlying conscious vision. We speculate that gating-by-inhibition in the visual cortex may provide a key foundation of consciousness.

Phosphene-guided transcranial magnetic stimulation of occipital but not parietal cortex suppresses stimulus visibility

Transcranial magnetic stimulation (TMS) applied over the occipital lobe approximately 100 ms after the onset of a stimulus decreases its visibility if it appears in the location of the phosphene. Because phosphenes can also be elicited by stimulation of the parietal regions, we asked if the same procedure that is used to reduce visibility of stimuli with occipital TMS will lead to decreased stimulus visibility when TMS is applied to parietal regions. TMS was randomly applied at 0-130 ms after the onset of the stimulus in steps of 10 ms in occipital and parietal regions. Participants responded to the orientation of the line stimulus and rated its visibility. We replicate previous reports of phosphenes from both occipital and parietal TMS. As previously reported, we also observed visual suppression around the classical 100 ms window both in the objective line orientation and subjective visibility responses with occipital TMS. Parietal stimulation, on the other hand, did not consistently reduce stimulus visibility in any time window.

Transient CNS Deficits and Migrainous Auras in Individuals Without a History of Headache

To demonstrate that benign transient focal neurological symptoms represent equivalents of migraine auras without headache. Benign focal neurological symptoms suggesting cerebral dysfunction are common and usually prompt an extensive diagnostic workup, but their etiology is often not elucidated. We hypothesized that benign transient focal neurological symptoms represent equivalents of migraine auras without headache, even in subjects who have never experienced migraine headaches. We led a cross-sectional study and identified individuals who presented at least 1 episode of unexplained transient focal neurological symptoms suggestive of cerebral dysfunction, but no history of migraine headache, among physicians and inpatients of an academic hospital. Cortical hyperexcitability, assessed by occipital transcranial magnetic stimulation (oTMS), was used as a marker of possible migraine auras without headache. Frequency of transient focal neurological symptoms suggestive of cerebral dysfunction among the physicians who responded was 9% (21/233), vs 0.09% (6/690) of inpatients. Most episodes resembled typical visual migrainous auras. Motor, sensory, and language dysfunction were more common among inpatients than among physicians. oTMS induced phosphenes in 12/16 (75%) subjects and in none of 10 controls. Benign focal neurological symptoms were common in our population and likely represent migraine aura without headache. Non-visual symptoms are less common and lead to medical consultation. oTMS is abnormal in most cases, supporting the diagnosis of migraine aura without headache and helping separate this benign condition from transient ischemic attacks.

Brain-Stimulation Induced Blindsight: Unconscious Vision or Response Bias?

A dissociation between visual awareness and visual discrimination is referred to as “blindsight”. Blindsight results from loss of function of the primary visual cortex (V1) which can occur due to cerebrovascular accidents (i.e. stroke-related lesions). There are also numerous reports of similar, though reversible, effects on vision induced by transcranial Magnetic Stimulation (TMS) to early visual cortex. These effects point to V1 as the “gate” of visual awareness and have strong implications for understanding the neurological underpinnings of consciousness. It has been argued that evidence for the dissociation between awareness of, and responses to, visual stimuli can be a measurement artifact of the use of a high response criterion under yes-no measures of visual awareness when compared with the criterion free forced-choice responses. This difference between yes-no and forced-choice measures suggests that evidence for a dissociation may actually be normal near-threshold conscious vision. Here we describe three experiments that tested visual performance in normal subjects when their visual awareness was suppressed by applying TMS to the occipital pole. The nature of subjects’ performance whilst undergoing occipital TMS was then verified by use of a psychophysical measure (d') that is independent of response criteria. This showed that there was no genuine dissociation in visual sensitivity measured by yes-no and forced-choice responses. These results highlight that evidence for visual sensitivity in the absence of awareness must be analysed using a bias-free psychophysical measure, such as d', In order to confirm whether or not visual performance is truly unconscious.

Direct injection of neural noise leads to double dissociation between accuracy and confidence

The relationship between accuracy and confidence in psychophysical tasks has traditionally been assumed to be mainly positive, i.e. the two typically increase or decrease together. However, recently we showed that spatial attention can lead to dissociations between visual sensitivity and confidence ratings (Rahnev et al., 2011, Nature Neuroscience). The data were explained by a computational model in which lack of attention increased the variability of the internal distributions. In order to test this model, we directly injected neural noise by applying transcranial magnetic stimulation (TMS) to the occipital cortex. We expected TMS to increase the variability of the internal response and, in a similar fashion as in inattention, lead to decreased accuracy but increased confidence. In our task, subjects discriminated between left- and right-tilted bars and simultaneously received single TMS pulses at SOA of 100 ms. Compared to TMS applied to a control site (vertex), occipital TMS led to significantly lower accuracy (p = .007) but significantly higher confidence (p = .01). This striking double dissociation provides strong support of our variance-based model. To confirm that the increased variability of the signal was indeed the key reason for the dissociation, we performed a formal model comparison analysis that used information theoretic methods. The analysis confirmed that our model was preferred over models in which TMS changed the signal strength or "dual-channel" models that postulate two processing streams. These results imply a central role of neural variability in perceptual decisions (Cohen and Maunsell, 2009) and suggest that confidence ratings may be produced by a highly suboptimal decision mechanism. Meeting abstract presented at VSS 2012

Sounds Reset Rhythms of Visual Cortex and Corresponding Human Visual Perception

SummaryAn event in one sensory modality can phase reset brain oscillations concerning another modality [1–5]. In principle, this may result in stimulus-locked periodicity in behavioral performance [6]. Here we considered this possible cross-modal impact of a sound for one of the best-characterized rhythms arising from the visual system, namely occipital alpha-oscillations (8–14 Hz) [7–9]. We presented brief sounds and concurrently recorded electroencephalography (EEG) and/or probed visual cortex excitability (phosphene perception) through occipital transcranial magnetic stimulation (TMS). In a first, TMS-only experiment, phosphene perception rate against time postsound showed a periodic pattern cycling at ∼10 Hz phase-aligned to the sound. In a second, combined TMS-EEG experiment, TMS-trials reproduced the cyclical phosphene pattern and revealed a ∼10 Hz pattern also for EEG-derived measures of occipital cortex reactivity to the TMS pulses. Crucially, EEG-data from intermingled trials without TMS established cross-modal phase-locking of occipitoparietal alpha oscillations. These independently recorded variables, i.e., occipital cortex excitability and reactivity and EEG phase dynamics, were significantly correlated. This shows that cross-modal phase locking of oscillatory visual cortex activity can arise in the human brain to affect perceptual and EEG measures of visual processing in a cyclical manner, consistent with occipital alpha oscillations underlying a rapid cycling of neural excitability in visual areas.

Distinct Causal Mechanisms of Attentional Guidance by Working Memory and Repetition Priming in Early Visual Cortex

Human attention may be guided by representations held in working memory (WM) and also by priming from implicit memory. Neurophysiological data suggest that WM and priming may be associated with distinct neural mechanisms, but this prior evidence is only correlative. Furthermore, the role of the visual cortex in attention biases from memory remains unclear, because most previous studies conflated memory and selection processes. Here, we manipulated memory and attention in an orthogonal fashion and used an interventional approach to demonstrate the functional significance of WM and priming states in visual cortex for attentional biasing. Observers searched for a Landolt target that was preceded by a nonpredictive color cue that either had to be held in WM for a later recognition test or merely attended (priming counterpart). The application of transcranial magnetic stimulation (TMS) over the occipital cortex modulated the impact of memory on search. Critically, the direction of this modulation depended on the memory state. In the WM condition, the application of TMS on validly cued trials (when the cue surrounded the sought target) enhanced search accuracy relative to the invalid trials (when the cue surrounded a distracter); the opposite pattern was observed in the priming condition. That the effects of occipital TMS on selection were contingent on memory context demonstrates that WM and priming represent distinct states in the early visual cortex that play a causal role in memory-based guidance of attention.

Direct injection of noise to the visual cortex decreases accuracy but increases decision confidence

The relationship between accuracy and confidence in psychophysical tasks traditionally has been assumed to be mainly positive, i.e., the two typically increase or decrease together. However, recent studies have reported examples of exceptions, where confidence and accuracy dissociate from each other. Explanations for such dissociations often involve dual-channel models, in which a cortical channel contributes to both accuracy and confidence, whereas a subcortical channel only contributes to accuracy. Here, we show that a single-channel model derived from signal detection theory (SDT) can also account for such dissociations. We applied transcranial magnetic stimulation (TMS) to the occipital cortex to disrupt the internal representation of a visual stimulus. The results showed that consistent with previous research, occipital TMS decreased accuracy. However, counterintuitively, it also led to an increase in confidence ratings. The data were predicted well by a single-channel SDT model, which posits that occipital TMS increased the variance of the internal stimulus distributions. A formal model comparison analysis that used information theoretic methods confirmed that this model was preferred over single-channel models, in which occipital TMS changed the signal strength or dual-channel models, which assume two different processing routes. Thus our results show that dissociations between accuracy and confidence can, at least in some cases, be accounted for by a single-channel model.

Causal evidence for subliminal percept-to-memory interference in early visual cortex

There has been recent interest in the neural correlates of visual short-term memory (VSTM) interference by irrelevant perceptual input. These studies, however, presented distracters that were subjected to conscious scrutiny by participants thus strongly involving attentional control mechanisms. In order to minimize the role of attentional control and to investigate interference occurring at the level of sensory representations, we developed a paradigm in which a subliminal visual distracter is presented during the delay period of a visual short-term memory task requiring the maintenance of stimulus orientation. This subliminal distracter could be either congruent or incongruent with the orientation of the memory item. Behavioral results showed that the intervening distracter affected the fidelity of VSTM when it was incongruent with the memory cue. We then assessed the causal role of the early visual cortex in this interaction by using transcranial magnetic stimulation (TMS). We found that occipital TMS impaired the fidelity VSTM content in the absence of the memory mask. Interestingly, TMS facilitated VSTM performance in the presence of a subliminal memory mask that was incongruent with the memory content. Signal detection analyses indicated that TMS did not modulate perceptual sensitivity of the masked distracter. That the impact of TMS on the precision of VSTM was dissociated by the presence vs. absence of a subliminal perceptual distracter and its congruency with the VSTM content provides causal evidence for the view that competitive interactions between memory and perception can occur at the earliest cortical stages of visual processing.